Got the Flies, Got No Reason

Sunday September 16, 2012


Mr Pest Control


The earlier this year I received a call to treat a house for flies. When I got there I did an inspection and found quite a few green flies all dead on the window ledge. I know that green flies associate with something dead. I also know that the green flies that go after a dead carcass are rather large, but the green flies I found are about half the size of the large ones. In any event I did a thorough inspection of the house, starting from the attic to the second floor and the first floor and finally the basement. I even checked the bottom of the furnace and the trap door at the bottom of the chimney. There wasn't anything dead in them. I've been back three times to treat the house and I can't find where they are coming from. The owner told me that they owned the house for two years and they had this problem from day one. They rent the house out and they have had other service techs there. No one has been able to solve this problem. I've spent hours treating every crack and crevice with chemicals labeled for flies and still no success. The first floor is where the majority of the flies are. Any ideas would greatly be appreciated.

Mr Pest Control


My first observation is with respect to your applications of insecticides into all those cracks and crevices. This probably was not justified since you had not really identified a need for it. I recognize that if blow flies are breeding indoors the maggots often squirm their way into gaps and crevices to pupate, but without really seeing any maggots or fly pupae, the use of a toxin in this case should have been avoided. It would be interesting to know what those "other service techs" may have used too, but I suspect that somewhere along the line someone spent time fogging the place just to kill the adult flies. This, of course, pretty much never resolves a fly infestation if the source is indoors. 

As to why these green flies are smaller than usual I suggest a couple of possibilities. First may be that they are not blow flies at all, but some other kind of metallic green fly or even bee, such as mining bees, long-legged flies, or cuckoo wasps. I mention this only as a remote possibility as all of these common insects would have no business indoors, so finding a couple could mean they just got in, but finding many of them repeatedly over a couple of years would be unlikely. It's also possible that these flies just happen to be smaller than normal, as sizes do vary within a species. 

Another possibility is that the flies are coming in from the outside, perhaps through doors or windows propped open with no screens in place. Blow flies can detect the wonderful odors of breeding sources, and quickly come indoors when the opportunity is presented. Besides dead animals they commonly breed in decaying vegetable matter, so filthy garbage cans and dumpsters are also common breeding locations. However, either dead animals or filthy garbage present for two years now should be pretty obvious with its odors and unless these tenants are, well, pigs, these breeding sources ought not to be there for so long. 

Another possibility that might draw blow flies is natural gas. Apparently the odor of methane gas from gas appliances and appliance vents closely resembles the odors given off by decaying carcasses. Perhaps these flies are finding their way inside through the roof vents of the appliances, and thus to the inside in some manner. It might be interesting to screen off all those exterior vent openings with screen with a mesh small enough to exclude flies, and see what happens. Maybe there is a gas fireplace with a leak, and the flies are coming down the chimney. 

Outdoor breeding sources might also exist that are producing large numbers of blow flies - rotting vegetation piles, dog feces, etc. - and this is providing plenty of flies to increase the odds that many will get inside. A close inspection of the exterior would be in order, including the garbage cans and the waste removal stream at this home. I once saw a garage filled with migrating maggots that were coming from a bag of household garbage that had not been properly disposed of for a couple of weeks. 

Mr. Pest Control

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